Los Angeles A mercury-based preservative once used in many vaccines does not raise the risk of neurological problems in children, concludes a large federal study that researchers say should reassure parents about the safety of shots their kids received a decade or more ago.
However, the study did not examine autism - the developmental disorder that some critics blame on vaccines. A separate study due out in a year will look at that issue, said scientists at the federal Centers for Disease Control and Prevention, who led the latest analysis and published results in Thursday's New England Journal of Medicine.
They found no clear link between early exposure to the preservative thimerosal and problems with brain function and behavior in children age 7 to 10. The results are in line with past research that found no connection between vaccines and neurological problems or autism.
Thimerosal has not been used in childhood vaccines since 2001, although it is still in some flu shots. The new findings apply to children immunized before then, or exposed to the preservative through shots their mothers received while pregnant. Thimerosal was put in vaccines to prevent contamination from bacteria.
Some doctors say the CDC study should reassure parents worried about the safety of vaccines. "It's good news for families," said Dr. Michael Goldstein, vice president of the American Academy of Neurology who works in private practice in Salt Lake City. "There's no evidence that these vaccines have caused injury."
The study involved 1,047 children who were exposed to varying levels of thimerosal while in the womb or after birth in the 1990s. The children belonged to four health maintenance organizations that are part of a federal project to study the side effects of vaccines. Their mercury exposure was determined through medical and immunization records and interviews with parents.
Each child was tested for speech and language skills, motor coordination and intelligence. Parents, teachers and trained specialists also rated stuttering, attention span and tic disorders such as head shaking, eye blinking and neck jerking. A total of 42 neurological problems were analyzed.
On balance, researchers did not find a consistent pattern between increasing thimerosal exposure and the risk of these problems. However, they said one finding merited further study: Boys exposed to higher mercury levels seemed to have more tic problems.